There are some applications (like touch screen controllers and radios) that are sensitive to supply noise, substrate noise, and electromagnetic interference (EMI), but, in many cases, this sensitivity is not permanent. This sensitivity can be limited to “quiet” periods. For example, touch screen controllers can perform a capacitance measurement from a touch screen or touch panel during short intervals or periods (i.e., about 100-200 μs) that are separated by “idle” intervals or periods (where, for example, back-panel biasing for picture display can be performed). During these capacitance measurements periods, any noise in supply lines or in substrate can significantly alter the measurement results, decrease sensitivity or even make touch detection impossible.
These applications are also usually the battery-powered applications, so efficiency of the system power management can be very important so as to lengthen battery life. To achieve efficiency, direct current (DC)-to-DC converters are generally used to provide power for system operations (i.e., operation of the touch screen controller) from a battery, and, oftentimes, these DC-to-DC converters are integrated on the die with system (i.e., touch screen controller) in order to decrease size and lower cost. However, these DC-to-DC converters are usually switched-mode converters (i.e., buck or boost converters) because these types of converters are more efficient that linear regulators (i.e., low dropout regulators or LDOs). Each time these switched-mode converters switches, though, noise is generated due to current spike(s) in the substrate and in power supply buses, especially in the ground bus. Voltage ripples on the output storage capacitor can also be generated, typically in order of 5-10 mV, at the switching frequency (usually between 1 and 5 MHz).
A common solution, to this problem, however, is to employ an LDO in conjunction with a switched-mode converter. An example of a system 100 that employs a buck converter 102 in conjunction with an LDO 104 (which provides power from a battery BAT to a powered circuit 106) can be seen in FIG. 1. As shown, the buck converter 102 is generally comprised of a plant (which is generally a driver circuit 110, transistors Q1 and Q2, an inductor L, and a capacitor Cl) and a controller (which is generally an error amplifier 112, voltage source 114, a pulse width modulator or PWM 108, and a voltage divider Rl/R2), and the LDO 104 is generally comprised of an amplifier 116, a transistor Q3, a voltage source 118, and a capacitor C2. In this example, voltage source 114 provides a reference voltage VDCREF to error amplifier 112 such that voltage VOUT (which is generated by buck converter 102 from battery BAT) is greater than reference voltage VLCOREF(by, for example 200-300mV) in order to decrease the power loss in LDO 104 (which is proportional to its dropout). In some cases, LDO 104 can suppress supply ripple by 40-50 dB, but LDO 104 does not does cure caused by current pulses in the ground bus, EMI, and the substrate noise.
Thus, there is a need for an improved power management circuit.
Some other examples of conventional circuits are: U.S. Pat. Nos. 6,873,322; 6,933,772; 7,148,666; 7,282,895; Sahu et al., “A Low Voltage, Dynamic, Noninverting, Synchronous Buck-Boost Converter for Portable Applications,” IEEE Trans. on Power Electronics, Vol. 19, No. 2, March 2004, pp. 443-452; and Lin et al., “Low-Dropout Regulators With Adaptive Reference Control and Dynamic Push-Pull Techniques for Enhancing Transient Performance,” IEEE Trans. on Power Electronics, Vol. 24, No. 4, April 2009, pp. 1016-1022.